EMI Suppression Filters (EMC and Noise Suppression)Noise Suppression Basic Course Section 1
Chapter 5 Conductor conduction and common mode

5-1. Introduction

The chapters so far have described the mechanisms of how noise occurs, how it is transmitted to and emitted from an antenna with reference to relatively simple models. However, in actual noise suppression measures, it is a rare case that the noise source has been directly connected to an antenna. In many cases, noise occurs in normal mode and is then converted into common mode. Subsequently it is transmitted through the ground of an electronic device and is then emitted through a cable or case as an antenna. Therefore, you need to consider the conversion from normal mode to common mode inside the noise transmission path.
It will be opposite when receiving noise. Many cases of noise intrusion occur in common mode. However, it eventually becomes normal mode when the circuit malfunctions or gets broken. In such a case, the conversion from common mode to normal mode is an issue. Since the mechanisms of noise emission and reception are the same, we only focus on the noise emission for the sake of explanation.
As shown in Fig. 5-1-1, this chapter first describes the existence of two modes (common mode and normal mode) for noise to be transmitted through a conductor and then describes the conversion into common mode. This normal mode is often expressed as differential mode. However, in order to avoid confusion with differential signal, it is referred to as normal mode for noise in this course.
Since the common mode noise is a complicated concept, the description here is partly based on our unique interpretation for the sake of simplifying the explanation. Please refer to technical books for accurate and detailed concepts ^{[References 1,2,3]} .

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5-2. Conductor conduction of noise

Since noise is an electric energy, it is naturally conducted through a conductor if it is connected. However, if the conductor forms a bunch just like a cable, the noise conduction is explained in two different ways: common mode and normal mode. Among these, common mode causes strong emission and reception of radio waves and has a complex mechanism, always causing troubles to engineers for noise suppression.
This section will first explain common mode and normal mode and then describe the basic configuration of EMI suppression filter for eliminating the noise. The occurrence of common mode will be explained in the next section as it involves a special concept.

5-2-1. Common mode noise

(1) Example of noise emission wherein a cable works as an antenna

The test in Fig. 5-2-1 reproduced the status that is often seen in the noise suppression for electronic devices.
An electronic device (before noise suppression) has been connected with an interface cable, and the noise emitted from the cable as an antenna has been measured. When there is no cable, the level is only low as shown in Fig. 5-2-1(a). However, when a cable is attached, the noise increases in the frequency range from 100MHz to 300MHz as shown in Fig. 5-2-1(b).
It is understood that in this status the noise emitted from the electronic device conducts from the connector to the interface cable and is then emitted from the cable as an antenna.

(2) Investigation of lines wherein noise conducts through

There is more than one wire inside a cable. Then, which line was the noise conducted through in the test of Fig. 5-2-1?
Generally, an interface cable contains a ground line, power line and signal line etc. Since the case of Fig. 5-2-1 is in fact a shielded cable, the noise might have been conducted through the shield as well. Therefore, a single wire having the same shape as the interface cable was attached to the terminal inside the connector where the respective lines have been connected, and then the noise was measured. The results are shown in Fig. 5-2-2. We chose a relatively slow-speed line for the signal line to show a typical case.
With reference to the results of Fig. 5-2-2, it is understood that whichever line it is connected, noise with more or less the same trend as Fig. 5-2-1(b) is emitted. When it is connected to the shield ground as shown in Fig. 5-2-2(d), noise is also emitted.
The results of Fig. 5-2-2 indicate that the common noise has been induced to all terminals in the connector that is connected to the cable. As above, the noise that is commonly conducted through the wires inside a cable is called common mode noise.

(3) Noise that has been superimposed over the ground is also called common mode noise

In contrast, the ground of electric circuit is generally considered as a reference point where there is the least noise. If noise has been superimposed over this ground as shown in Fig 5-2-2(c) or (d), the same noise will be superimposed over the power source and signal. Therefore, the noise that has been superimposed over the ground is sometimes referred to as common mode noise.
Although common mode noise is an ordinary problem to be handled by noise suppression, it is a component that has complex concept and mechanism and it is hard to explain it logically. First, this section will describe how the component of common mode is transferred, and then the next section will describe the mechanism of causing common mode noise.

5-2-2. Two modes for noise conduction

(1) Common mode and normal mode

Electric circuits are based on an electric current that makes a round trip along the path. If a part of this circuit is extracted as a cable as shown in Fig. 5-2-3(a), the cable is represented by two wires on which an electric current goes out and comes back. These currents are flowing in the opposite direction to each other with the same magnitude. Therefore, the total will be always zero. This way of current flow is called normal mode.
In contrast, the currents may flow in the same direction on the lines inside a cable as shown in Fig. 5-2-3(b). It is called common mode. The common mode is a component of electric current that flows in the same direction as shown in the figure due to the same voltage applied to each line in some form. As shown in the figure, it is understood that this current is caused by a current that has leaked out via the floating electrostatic capacitance of the load retained against the earth and has then returned to the noise source via the earth. (The current may be caused by a direct connection between the load and noise source without going through the earth)

(2) In case of many lines

Even though the circuit becomes complex with many wires inside cables that are sharing the ground, as long as there is no detour or leakage of the current, the sum total of the current in the entire cable becomes zero as shown in Fig. 5-2-4(a). This kind of status is also called normal mode. If there are many lines as above, the magnitude of the current of each line is not necessarily the same.
Fig. 5-2-4(b) shows the current wherein common mode is applied to the same circuit. In this case, the wires have a current in the same direction with the same voltage with reference to the earth. That means the line voltage is zero in common mode. Therefore, it tends to be difficult to observe common mode noise with use of ordinary measuring devices such as an oscilloscope.
Eventually, the current that flows on each line is a total current of normal mode and common mode. Although we can provide a clear description based on the figure, it is usually very difficult to separate these two currents from the current that flows on each line. Therefore, it is important to speculate the noise mode that flows by devising the observation method for the purpose of noise suppression.

5-2-3. Normal mode and differential mode

(1) Normal mode is also called differential mode

Normal mode on two lines as shown in Fig. 5-2-3 is sometimes called differential mode. Since the cases handled here also include the case with many lines as shown in Fig. 5-2-4, it is generally referred to as normal mode, and only if we refer to a pair of electric wires (just like a differential signal), it will be referred to as differential mode.

(2) Normal mode is also used for circuit operations

Normal mode and common mode are also used for circuit operations and signal transmissions apart from noise conduction. Usually normal mode is used as it is described as signal source in Fig. 5-2-3.
In recent years, differential signals have been used in many of circuits that transmit high-frequency signals. Differential signals carry signals in differential mode (normal mode) just as the name indicates. However, since other signals are sometimes superimposed over transmission, common mode may also have been used in multiplex. In this case, the cable needs to be shielded in order to prevent common mode from being emitted and turned into noise.

5-2-4. Effect on noise emission

(1) Normal mode noise emission

When noise is conducted through a cable, normal mode will cause a very small amount of noise emission. This is because the electromagnetic fields respectively caused by the going and returning currents cancel with each other at the observation point as shown in Fig. 5-2-5. In order to reduce the emission, a twisted pair or shielded cable may be used for the cable section.
The printed board to which this cable is connected has a wider gap between the wires as shown in Fig. 5-2-5. Here, the cancelling effect of the going and returning currents is reduced and the wiring works like a loop antenna. Therefore, noise that corresponds to the loop area is emitted from this section despite normal mode.
Even if the cable has not been connected, the current that operates a circuit as shown in Fig. 5-2-6 is normal mode and the wiring that makes the circuit forms a loop antenna, causing noise emission in the same way. Therefore, in order to reduce the noise emitted from a printed board, the pattern needs to be designed so that the area of the current loop is reduced. Using a ground plane with use of a multilayer board allows reducing the area of the current loop as the current can flow directly below the signal line.

(2) Common mode noise emission

Unlike normal mode, when noise is conducted through a cable in common mode, the cancelling effect cannot be obtained. As shown in Fig. 5-2-7, the electromagnetic fields created by respective currents are intensified with each other at the measurement point. Therefore, with the same current flowing, common mode can emit radio waves that are way stronger than those by normal mode (can be about 1000 times higher). Therefore, it is important to suppress the common mode current for reducing noise emission.
Since the common mode current usually flows via floating electrostatic capacitance as shown in Fig. 5-2-7, it will not be a large current in the low frequency range due to its high impedance. However, in the high frequency range wherein the entire structure works as an antenna, the emission by common mode tends to be intensive since the current can easily flow due to its reduced impedance.
In addition, the normal mode current is a current mode that is used for the circuit operations, it is not possible to completely eliminate it by a filter. In contrast, common mode is a component that is usually unnecessary and thus can be eliminated by a filter as much as you need. The configuration of noise filter will be described below.

5-2-5. Configuration of noise filter

(1) Forming a low-pass filter with capacitors and inductors

Generally, capacitors (C) and inductors (L) are used to form a low-pass filter in the middle of or at a connection point of cable that works as a noise transmission path in order to block out noise conduction. Since Chapter 6 will describe low-pass filters in detail, this chapter will only explain the basic filter configuration.

(2) Filter for normal mode

As shown in Fig. 5-2-8, a filter for normal mode can be formed by inserting a capacitor between lines and attaching impedance elements (choke coil or ferrite bead etc.) in series.
The current of normal mode noise is in the same direction as the current used for the circuit operations. Therefore, when eliminating noise by the filter, some of the components needed for the circuit operations are also eliminated at the same time. The values of L and C are adjusted so that the cut-off frequency of the low-pass filter does not go over the components needed for the circuit operations.
In addition, as describe in Fig. 5-2-8, how to use impedance elements varies depending on the conditions of circuits and cables. If all lines are floating with reference to the ground just like commercial power lines, the circuit is considered as a balanced circuit and an impedance element is used for both lines. In so doing, you need to maintain the balance so that the impedance is the same.
If one side is grounded such as a case of digital circuit, the circuit is considered as an unbalanced circuit and no impedance element is normally used for the ground. However, if noise has been induced to the ground (that means, common mode noise has been induced), an impedance element may be used on the ground side as well.
Here, the terms "balanced" and "unbalanced" refer to how the voltage is retained with reference to the earth when conducting normal mode through. If the voltage is symmetrically applied to two lines, it is referred to as balanced, and if it is concentrated on one line, it is referred to as unbalanced. The other line of the unbalanced circuit is the ground to which almost no voltage is applied.

(3) Filter for common mode

As shown in Fig. 5-2-9, a filter for common mode can be formed by connecting capacitors to the ground (referred to as Y capacitor). You should try to use a common mode choke coil for the impedance element as much as possible. If there are a number of wires in the cable, it is effective to create a kind of common mode choke coil by turning the cable around a ferrite core or sandwiching the cable with a ferrite core as shown in Fig. 5-2-10. Common mode choke coil will be described in detail in the next chapter.
When common mode noise appears, the noise may show up on the ground to which the Y capacitor is connected. In this case, the effect of Y capacitor is reduced since the Y capacitor has not been connected to an appropriate ground.
In such a case, a ground point to which the Y capacitor is connected needs to be created separately. As shown in the figure, the wiring of this ground is intended to form a returning path of the noise for the noise source.

(4) Filter that is effective for both common mode and normal mode

Noise filters that are used for commercial power lines generally provide measures for noise that is a mixture of common mode and normal mode and thus comprise a filter that can deal with both modes. Fig. 5-2-11 shows a typical circuit configuration ^{[Reference 4]} . This example shows a common mode choke coil for the impedance element. However, if the normal mode noise is high, the impedance may be insufficient and thus the filter can be used by adding a choke coil for normal mode.

5-2-6. Example of noise suppression by filter

(1) Noise that is conducted through commercial power line

Although Fig. 5-2-1 presented a measurement example of noise emitted from the interface cable of electronic device, the conduction of noise in a relatively low frequency range becomes an issue in the power lines of electronic devices. Common mode and normal mode become issues in the power lines.
One of the typical noise sources that emit noise to the power lines is switching power supply. Fig. 5-2-12 shows an example of noise measurement for a switching power supply.
The noise measurement on AC power lines attaches a type of probe for measuring noise called LISN (Line Impedance Stabilizing Network) to the power lines as shown in Fig. 5-2-12(a) and measures the noise that is conducted through the power lines. The measurement has been carried out without the built-in noise filter of the switching power supply. The measured frequency range is 150kHz to 30MHz and a spectrum analyzer is used for the measurement of peak detection.
As shown in the measurement result of Fig. 5-2-12(b), strong noise has been observed at the integral multiples of 150kHz, which is the switching frequency of the switching power supply. Since the frequency axis of the graph is logarithmic, the noise intervals seem smaller in the high frequency range over 1MHz. However, if you have a closer look, you can see that the intervals in this range are also 150kHz.

(2) Separation of noise mode

The measurement result shown in Fig. 5-2-12 has measured the voltage to the ground for each line. Although it shows Va and Vb, you can see that the same level of noise has been observed on both lines. This is observing the mixture of common mode and normal mode. Usually the noise regulations set a limit for this voltage.
If you use a certain LISN (e.g. LISN that supports CISPR 16), the common mode and normal mode in the noise can be separately observed. Fig. 5-2-13 shows those separated from the measurement result of Fig. 5-2-12. In the figure, Sym (Symmetry) represents normal mode, while Asym (Asymmetry) represents common mode.
The measurement result of Fig. 5-2-13 indicates that the normal mode is high in the lower frequency range and the common mode is high in the higher frequency range in this switching power supply. This trend is generally seen in switching power supplies.

(3) Checking the effect of noise filter

Fig. 5-2-14 shows the results of checking how respective parts of the noise filter shown in Fig. 5-2-11 work for the noise of the switching power supply shown in Fig. 5-2-13.
Fig. 5-2-14(a) shows the measurement result wherein all parts shown in Fig. 5-2-11 have been attached. Noise has been very well suppressed in comparison with Fig. 5-2-13(b) that does not use the parts.
Figs. 5-2-14(b) to (d) show the results of taking out the parts of the noise filter shown in Fig. 5-2-11 one by one. It is understood that X capacitor is mainly effective for normal mode, Y capacitor is mainly effective for common mode, and common mode choke coil is effective for both modes. Therefore, it was confirmed that these three parts are indispensable to eliminate noise that is a mixture of normal mode and common mode as shown in this example.

(4) Effect of each part can be easily seen by taking it out after noise has been completely eliminated

Generally, the effect of noise suppression may not be successfully observed even if each part is attached one by one as any change in faint noise has been hidden in strong noise. Therefore, first creating the status of suppressing noise as shown in Fig. 5-2-14(a) and then taking out the parts one by one so that you can check the effect of each part, and thus you can easily determine the effect and necessity of each part. This method is useful not only for checking conducted noise, but also for checking the effectiveness of the parts when taking measures for emitted noise.
Although you may feel it is different from what you expected, the common mode choke coil is also playing a role for eliminating the normal mode noise in Fig. 5-2-14(c). This is because the common mode choke coil contains a small amount of inductance for normal mode. When using a common mode choke coil for power supply, its small amount of inductance sometimes makes an effect on normal mode in this manner. The details will be described in the explanation for common mode choke coil.

5-2-7. Common mode noise on differential signal

(1) Transmission of differential signal

In recent years, differential signals have been more commonly used for the high-speed digital transmission such as USB. Differential signals contain common mode noise, which is slightly different from those explained so far.
Differential signal applies a reverse phase signal to each line of a line pair as shown in Fig. 5-2-15 and receives a signal as a line voltage on the receiver side. If these two currents are symmetrical with each other, the current components are only normal mode, and thus very small noise occurs due to the mechanism shown in Fig. 5-2-5.
Furthermore, in case of receiving a noise induction from the outside, it is less likely to be affected. As described later, this is because the noise induced from the outside to the cable is common mode and thus would not cause any voltage between the lines of the receiver.

(2) Common mode noise that occurs in differential signal

However, if there is a slight imbalance in the signal that has been transmitted by the two lines, the unbalanced components turn into common mode. As shown in Fig. 5-2-16, the factors that are considered to cause imbalance are:

1. (a) Time deviation of rise or fall
2. (b) Speed deviation of rise or fall
3. (c) Magnitude deviation of voltage or current
4. (d) Superimposed common mode noise

You may say that (a) to (c) are issues in forming a signal waveform rather than saying noise problems (referred to as Signal Integrity: SI). Such an imbalance in signal waveform occurs due to a difference in the wire lengths, a bent in the wires, or a difference in the impedances of terminating resistors, other than any problems in the driver or receiver IC. As described above, the common mode noise occurred due to an imbalance in the signal waveform is observed in the form of harmonics of the signal frequency in the noise spectrum.
(d) is often seen when noise has been applied from the outside to the power supply or ground of the driver or receiver. Although the noise may seem like signal harmonics, it can occur at a frequency that is not at all related to the signal frequency.
If these components are conducted through the cable, a common mode current occurs and thus results in a cause of noise emission.

(3) How to suppress noise in differential signal

As shown in Fig. 5-2-17, common mode choke coils are used to block out such a common mode current and suppress any imbalance in the signal waveforms as shown in Fig. 5-2-16(a) to (c) for differential signals. Usually it is used on the driver side. However, if noise occurs on the receiver side, it is also used on the receiver side.
Parts with a small attenuation for differential mode are chosen for the common mode choke coils used here so that those do not adversely affect the differential signal.
In addition to the common mode choke coils, a shielded cable is also used for noise suppression in differential signal. Two coaxial cables may be used for the signal pair section.
It is also good to use a common mode choke coil or shield in the signal pair section for the noise of Fig. 5-2-16(d). However, it is more effective to use an EMI suppression filter for the power supply of the driver or receiver IC as shown in Fig. 5-2-17.

5-2-8. Noise reception and mode conversion

(1) Noise becomes common mode when received by cable

So far the case of emitting noise from a cable has been described. As opposed to this, when a cable receives noise, it generally means that noise has been induced in common mode to the wires inside the cable as shown in Fig. 5-2-18.
In case of common mode, the line voltage is zero, and if the signal is received by the line voltage as shown in the figure, the electric circuit can operate without any problems. Therefore, even if a cable receives noise, it would not cause noise interference as long as the receiver operates by voltage.

(2) Conversion of noise mode

However, in the real world, when noise enters a cable, various interferences occur. One old example is interference from radio broadcasting to telephone voice due to radio waves entering telephone lines. Why does this kind of interference occur?
In many of these cases, common mode has been converted to normal mode at the point where the cable is connected to the circuit. If there is a difference between the impedance of each line ( Z ₁ ) and the impedance of the earth ( Z ₂ ) as shown in Fig. 5-2-19(a), it creates a difference in the common mode voltage that the receiver receives, causing a noise voltage between the lines. In such a case, you may say that common mode has been partly converted into normal mode ^{[Reference 1]} .

(3) Unbalanced terminal impedances cause mode conversion

These Z ₁ and Z ₂ do not mean that these parts actually exist, and these represent the impedances formed by floating electrostatic capacitance etc. Therefore, if terminating resistors for which the impedances have been aligned in advance are attached at these points, the conversion to normal mode may be reduced.
As shown in Fig. 5-2-19(b), if the signal is received by a circuit that has been grounded on one side, a half of the noise will be converted into normal mode. That means noise can easily enter an unbalanced receiver circuit such as a digital circuit. When connecting a cable to such a circuit, a filter circuit described later is required.

(4) Mode conversion may occur inside IC

Even if conversion to normal mode does not occur, if common mode is powerful, it can be converted to normal mode inside the receiver IC. The performance of IC to eliminate common mode is indicated by an index called CMRR (Common-Mode Rejection Ratio).
In order to prevent the conversion to normal mode, the values of the terminating resistors are matched with each other as shown in the figure so that no difference is caused between the impedances with reference to the ground. In addition, an IC with high CMRR is chosen for the receiver.

(5) To prevent mode conversion

When a balanced cable such as a phone line, LAN cable or power code is connected to an electric circuit, the noise mode can be easily converted as shown in Fig. 5-2-19(b) as many of such electric circuits are unbalanced circuits. In order to prevent this from happening, there are two methods as follows:

1. (i) Use a balun transformer or common mode choke coil etc. to provide the balanced-unbalanced conversion so that the impedance balance can be maintained.
2. (ii) Remove the generated normal mode noise with a noise filter

(i) is a method of inserting a balanced-unbalanced conversion circuit between the cable and circuit as shown in Figs. 5-2-20(a) and (b). Such a circuit is used for connecting a communication cable.
(ii) uses a capacitor and impedance element (ferrite beads) as shown in Fig. 5-2-20(c). Although this is a makeshift solution, it can eliminate noise interference with relatively inexpensive parts.

5-2-9. Characteristics of common mode and normal mode

(1) Normal mode noise occurs depending on circuit operations

When an electric circuit is operating, the current flows in normal mode. Therefore, the normal mode noise naturally occurs due to the circuit operations. For example, a surge caused by turning a power switch on and off, or a harmonic component contained in a digital signal causes normal mode immediately after it occurs.
It is understood that when there is a slight imbalance in the current inside the noise transmission path, the component appears as common mode.

(2) Shield may not be useful for common mode noise

To make a shield work (particularly in case of electrostatic shield), it needs to be connected to the ground. However, if common mode noise occurs, the noise often goes over the ground of the shield as well. Therefore, the common mode current also flows through the shield and emits noise from the shield working as an antenna.
As above, noise cannot be shielded by connecting the shield to the ground where common mode is conducted through. In order to make the shield work, you first need to create a reliable ground. This is the reason why the noise suppression of common mode is very difficult.

(3) How to shield common mode noise

To make a robust ground that makes the shield work, a shielding case is created in the manner to enclose the noise source and floating electrostatic capacitance as shown in Fig. 5-2-21 and then the case itself is used as a ground. (It is called "Faraday cage")
In so doing, the returning path of the common mode current goes through the shield instead of the earth. In this status, the common mode noise is considered to have been eliminated. This is because the sum total of the currents becomes zero as the entire cable is viewed including the shield.
Although such a shield configuration is ideal, it is generally large scale and expensive.

(4) Where is common mode connected to?

Regarding the connection point for the common mode noise source or floating electrostatic capacitance in Fig. 5-2-3(b), it is not necessary that there is a particular connection point inside the circuit. However, since the point is usually the largest in size in the circuit and becomes a reference point, you may consider that it is equivalent to the ground connection.
Therefore, in the status of having a voltage with reference to the ground, you may say that common mode noise has been induced.

(5) Observation of common mode and normal mode

You can determine whether or not common mode noise is flowing in a cable by using a current probe that can grip the entire cable. A normal mode current would not cause any output from such a current probe.
In contrast, common mode noise always has zero in line voltage. Therefore, when measuring the line voltage with a differential probe, it is measuring the normal mode voltage by excluding common mode.

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“5-2 Conductor conduction of noise” - Key points